Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater, say 1000 meters or more) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 meters. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers.
Unbonded flexible pipe has been used for deep water (less than 3,300 feet (1,005.84 meters)) and ultra deep water (greater than 3,300 feet) developments. It is the increasing demand for oil which is causing exploration to occur at greater and greater depths where environmental factors are more extreme. For example in such deep and ultra-deep water environments ocean floor temperature increases the risk of production fluids cooling to a temperature that may lead to pipe blockage. Increased depths also increase the pressure associated with the environment in which the flexible pipe must operate. As a result the need for high levels of performance from the layers of the flexible pipe body is increased.
Flexible pipe may also be used for shallow water applications (for example less than around 500 meters depth) or even for shore (overland) applications.
In flexible pipes there are often used polymer layers, such as PVDF (polyvinylidene fluoride), that may be formed by extrusion. Most polymers will have a certain maximum allowable strain above which the risk of damage to the material is much greater. In flexible pipes where a polymer layer lies adjacent an armour layer (such as a polymer barrier layer adjacent a metallic pressure armour layer), the polymer layer may be subjected to quite severe non-uniform, highly localised strain. This is because the armour layer is usually formed from interlocking wires of certain cross section, and there are certain gaps between adjacent windings. The polymer layer tends to deform and creep into the gaps when under pressure.
U.S. Pat. No. 4,706,713 describes a pipe having a PVDF layer.
In accordance with industry regulations, all flexible pipe structures must undergo a factory acceptance test (FAT) prior to sale. This involves pressurising a pipe bore with a fluid such as water at 1.5 times the usual pressure of use. The water is thus a pressurising medium.
The application of internal pressure (i.e. pressure from within the bore) to the pipe produces radial expansion in all layers and this is when a polymer layer undergoes deformation and tends to creep into the gaps of an overlying armour layer. At high pressures (about 8000 psi/55 MPa or more), the resultant strain distribution within the polymer layer can be highly localised at the areas around the gaps, and the polymer material may deform by cavitation rather than plastic flow. This can in turn result in the formation of microcrazing or microcracking on the radially inner surface of the polymer layer. During any subsequent loading (such as the loading experienced during normal use in transporting production fluids) this microcrazing may then extend to form longer/deeper cracks throughout the polymer layer. This increases the risk of failure of the polymer layer and may ultimately lead to loss of pressure containment, having an adverse effect on the lifetime of a flexible pipe.